1. Field of the Invention
The invention relates to a method of producing hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in a diluted aqueous solution of mineral acid in the presence of platinum catalysts suspended on a support in multiple subsequent reaction stages.
2. Background Art
Continuous production of hydroxylammonium salts by catalytic reduction of nitrogen monoxide with hydrogen in a diluted aqueous solution of mineral acid in the presence of suspended noble metal catalyst is a known method used at industrial scale and has been described in patent specification DE 1 177 118. According to this document, an aqueous solution of mineral acid that contains the suspended catalyst is typically conducted through multiple subsequent reaction stages (cascades), a mixture of nitrogen monoxide and hydrogen is introduced to each reaction stage, and the catalyst-containing hydroxylammonium salt solution is removed in the last reaction stage.
Although this method has proven its worth in principle, there is a need to increase the capacity of existing plants and to minimize the formation of undesired by-products such as dinitrogen monoxide, nitrogen and ammonia salts which reduce the yield of hydroxylammonium salt. A high concentration of dinitrogen monoxide also results in explosive mixtures. The formation of foam on the surface of the reaction mixture has an adverse effect in this context.
Accordingly, many studies aimed at improving the method have been conducted. Patent specification DE 2 736 906 B1 describes a method for producing hydroxylammonium salts in which an increased amount of platinum catalyst supported by graphite speeds up the reduction reaction so that the space-time yields of nitrogen monoxide is considerably higher (meaning higher throughput).
The disadvantage of this method is increased loss of platinum. In addition, the quantity of the catalyst cannot be increased indefinitely as this would jeopardize the required flow and filtering properties of the suspension.
Patent specification DE 3 713 733 describes a method for producing hydroxylammonium salts in which the formation of by-products is mainly suppressed by using supported platinum catalysts partially poisoned with sulfur and selenium, and in which the metallic platinum is precipitated from aqueous platinum solutions on supporting material using reducing agents in the presence of organic chelating agents.
The disadvantage of this method is that catalyst production becomes unjustifiably complicated.
Patent specification DE 3 130 305 A1 describes a method of producing hydroxylammonium salts in which the formation of foam on the surface of the reaction mixture is prevented or at least considerably suppressed by limiting the fine-grain portion of the supported platinum catalyst or by sprinkling the surface of the reaction mixture with reaction mixture and/or freshly supplied mineral acid or by adding foam-inhibiting compounds.
The disadvantage of this method is that a high fine-grain portion as compared to the threshold value forms relatively fast into the use life of the catalyst or that sprinkling the surface of the reaction mixture requires a relatively great technological effort. In addition, adding foam-inhibiting foreign substances often has to be ruled out for reasons of process stability and product quality in the subsequent processing of hydroxylammonium salts, e.g. when producing caprolactam.
The supply regime of mineral acid, for which various variants were developed, has played an important part in studies on capacity increase by means of a high reaction speed and increasing the selectivity of the conversion into hydroxylammonium salts, i.e. suppression of by-product formation.
Patent specification DE 3 107 702 describes a method of producing hydroxylammonium salts in which a defined pH value is set for the last reaction stage and in which the measured pH value controls the supply of fresh aqueous mineral acid to the first reaction stage. In this way, the formation of explosive exhaust gas mixtures or the increased formation of by-products in the last reaction stage is prevented.
Yet another method of producing hydroxylammonium sulfate described in patent specification DE 4 132 800 splits the supply of sulfur advantageously in such a way that diluted sulfuric acid is supplied to the first reaction stage and concentrated sulfuric acid is supplied to one or more of the subsequent stages while the content of free sulfuric acid is considerably decreased in the last reaction stage.
A similar method for producing hydroxylammonium salts is known from patent specification DE 10062325 according to which the supply of diluted aqueous solution of mineral acid is split into at least two partial streams while the supply is controlled using preferably the pH value of the last reaction stage to which the second partial supply stream is added.
The three patented methods relating to acid supply mentioned above are unsatisfactory despite their advantages because they make plant engineering more complicated and tend to impair process stability if the measured pH value is used as a control variable.